This invention relates to pneumatic servo boosters and, particularly, to pneumatic servo boosters of the kind including a shell housing comprising a front and rear shell, a valve body incorporating therein a valve mechanism and being slidably supported by the rear shell, and a partitioning member cooperating with the valve body for partitioning the interior of the shell housing into front and rear chambers.
Pneumatic servo boosters of the kind aforementioned are widely used in vehicle hydraulic braking system and, usually, a master cylinder is connected to the front shell, and the master cylinder and the servo booster are mounted on a toe-board, which partitions the driver's room of a vehicle from the engine compartment, by a plurality of bolts secured to the rear shell of the servo booster. In such case, the force acting on the master cylinder in generating hydraulic pressure in the master cylinder by e.g. depressing a brake pedal is transmitted to the toe-board through the front and rear shells. When the strength of the shell housing is not adequate for preventing the shell housing from deforming due to the force acting on the master cylinder, the shell housing will be deformed, thereby decreasing the effective stroke of the servo booster.
The aforementioned problem can be solved by increasing the thickness of the front and rear shells or by using stronger material for the shell housing; however, either of these solutions will increase the cost of the shell housing.
There has been proposed providing at least one, preferably two or three, connecting rods in the shell housing extending through the front and rear chambers and through the partitioning member with the opposite ends thereof being connected respectively to front and rear shells. The connecting rod can effectively transmit the force acting on the master cylinder to the rear shell and, accordingly, to the toe-board, thereby minimizing the deformation of the shell housing.
The connecting rod passes through the partitioning member and is displaceable relative thereto; thus, there is another problem in that it is difficult to have an effective seal between the connecting rod and the partitioning member.